This works for sure? I would not expect this to work in plain bash (I don't know how makefiles work though so I am very likely wrong)

Bash test (Cygwin)
```bash
dylang@DylangHome ~
$ ls

dylang@DylangHome ~
$ mkdir -p $(INSTALLDIR)
-bash: INSTALLDIR: command not found
mkdir: missing operand
Try `mkdir --help' for more information.

dylang@DylangHome ~
$ export INSTALLDIR='bananas'

dylang@DylangHome ~
$ mkdir -p $(INSTALLDIR)
-bash: INSTALLDIR: command not found
mkdir: missing operand
Try `mkdir --help' for more information.

dylang@DylangHome ~
$ mkdir -p $INSTALLDIR

dylang@DylangHome ~
$ ls
bananas
```